In vitamin K deficiency and after coumadin administration paraprothrombin, a circulating prothrombin molecule which cannot be activated to thrombin by factor Xa, occurs in the blood of man and animals. The site of vitamin K action in the formation of a functional precursor of thrombin must be thus manifested in the difference between normal prothrombin and paraprothrombin. This difference will be sought by the sequencing of human prothrombin and paraprothrombin, with emphasis on the "pro" piece, i.e. that part of the prothrombin molecule lost in the activation of of prothrombin to alpha-thrombin. This appears to be the portion of the molecule which contains the vitamin K dependent site. Since our data indicate that the vitamin K dependent step may involve the incorporation of carbohydrate into prothrombin, the effect of vitamin K administration to deficient animals on the incorpoation of labeled carbohydrate into prothrombin as compared with paraprothrombin will be studied. Vitamin K, being a quinone, reacts with the sulfhydryl group, and may thus affect the final conformation of prothrombin activable in the presence of factor Xa. Both human and animal prothrombin and paraprothrombin will be examined and compared as to S-S bonding. Microsomal supernatant, prepared from normal liver, but not from vitamin K deficient liver converts, in vitro, an inactive precursor to active factor X. The enzyme responsible for this vitamin K dependent reaction will be purified from the supernatant and studied as to the role played by vitamin K. The metabolically active form of vitamin K will be further investigated. Understanding of the prothrombin molecule and its vitamin K dependent formation is the key factor in the understanding of thrombin formation and thus of normal hemostasis, thrombosis and the action of the oral anticoagulants in man.